This invention relates to a surge absorber used for protecting communication equipment, such as, facsimiles, telephone switchboards, modems, and the like, from transient surges generated into the communication lines by lightning, and the like. More particularly, it relates to a surge absorber which includes a surge absorbing element for protecting communication equipment from transient surges and a thermal response switch which is connected in parallel to the surge absorbing element and which prevents abnormal and deleterious heating of the surge absorbing element when continuous overvoltages or overcurrents flow into the surge absorber.
Surge absorbing elements are conventionally classified into two types, i.e., 1) semiconductors, such as, ZnO varistors, SiC varistors, zener diodes, etc.; and 2) gas discharge tubes, such as, air gap discharge tubes, micro gap discharge tubes, etc.. As compared to type 2), type 1) can easily respond to transient surges. However, type 1) cannot accommodate a larger surge current because it has a higher resistance than type 2).
Such a surge absorbing element is connected in parallel with a pair of input lines to the electronic device to be protected and is designed to operate at a higher voltage than the operating voltages of the electronic device. The surge absorbing element is a resistor having a high resistance when the voltage applied thereto is lower than the discharge starting voltage or striking voltage thereof, and is a resistor having a low resistance of tens of ohms when the voltage applied thereto is higher than the discharge starting voltage or striking voltage thereof.
Accordingly, when surge voltages, such as, lighting surges, etc., are instantaneously applied to an electronic circuit including the surge absorbing element and the electronic device, the surge absorbing element conducts the surge voltages and serves to protect the electronic device from the surge voltages. However, when an overvoltage or overcurrent, e.g., due to an accident, is continuously applied to the electronic circuit, a certain amount of current continuously flows through the surge absorbing element. This results in the surge absorbing element being heated to high temperatures. The heat radiating from the surge absorbing element can cause the protected electronic device as well as electronic devices surrounding the surge absorbing element to be damaged and/or catch fire.
The semiconductor type of surge protecting element is particularly prone to severe overheating upon the application of a continuous overvoltage or overcurrents due to its higher resistance. This can result in deterioration of the performances, destruction of the element, and/or, catching fire of the protected electronic device, as well as electronic devices surrounding the semiconductor type element. Also, ZnO varistors in semiconductors generate high remaining voltages which can lead to thermal damage to the varistors and electronic circuits connected to the varistors.
A typical example would be an accident wherein the communication lines of the communication equipment contact the AC power lines. While it does not usually happen that such accidental overvoltages or overcurrents are continuously applied to the surge absorbing element, to achieve maximum safety, it is now recognized that additional safety measures should be taken to avoid such accidental problems and the potential fires caused thereby. As an example, the UL (Underwriter's Laboratories Inc.) of the U.S.A. prescribes a safety standard for surge absorbing elements so that they do not cause fire or electrical shock in electronic devices surrounding the surge absorbing element when continuous overvoltages or overcurrent are applied.
A surge absorber capable of passing the above safety standard is disclosed in Japanese laid-open patent application No. S63-18923. Shown therein is a surge absorber comprising a gas discharge tube and a low melting point metal wire which is connected in series to the gas discharge tube and is adhered to the surface of the gas discharge tube. When an overvoltage or overcurrent, caused by the accidental contact of the communication lines with the AC power lines, is continuously applied to this surge absorber, the series connected low melting point metal wire is blown by the heat radiating from the surface of the gas discharge tube. This disconnects the current flowing to the gas discharge tube and prevents the abnormal heating of the gas discharge tube. However, this surge absorber has the disadvantage that it must be replaced whenever the low melting point metal wire is blown in order to immediately restore the electronic circuit connecting the surge absorber. This replacement procedure is time-consuming and troublesome. If a low melting point metal wire having a larger thermal capacity is used to avoid the problem, the low melting point wire can not be blown and the gas discharge tube is then heated to abnormally high temperatures, results in the electronic device catching fire.